1. Field of the Invention
Embodiments of the present invention generally relate to the field of photovoltaic device manufacture and, more specifically, to a method for making crystalline silicon solar cells on low purity substrates.
2. Description of the Related Art
Solar cells are photovoltaic devices that convert sunlight directly into electrical power. The most common solar cell material is crystalline silicon, which is in the form of single or multicrystalline substrates, commonly referred to as wafers. Because the amortized cost of forming crystalline silicon-based solar cells to generate electricity is higher than the cost of generating electricity using traditional methods, there has been an effort to reduce the cost to form solar cells.
The present invention relates to the development of methods to enable the use of low-cost silicon materials for use in solar cells. Generally, in order to obtain good efficiency in crystalline silicon (c-Si) solar cells, high purity polysilicon is used as a starting material. The purity of the silicon is related to the ability of electrons to travel freely and therefore affects efficiency in relation to the conversion of photons in the solar process. Although the level of purity of the silicon required for solar cells is not as great as that necessary for electronic grade silicon used in semiconductor fabrication (close to 99.9999999%, with impurities in the parts per billion range), a high degree of silicon purity (99.9999% or better, with maximum impurity concentration in the parts per million range) is still required.
Typically, high purity polysilicon is obtained by purifying metallurgical grade silicon (MG-Si) using the “Siemens process.” The Siemens process for purifying silicon is commonly used because it removes boron and phosphorus impurities from silicon better than other known methods. In the Siemens process, high-purity silicon rods are exposed to trichlorosilane (or silane) at 1150° C. The trichlorosilane (or silane) gas decomposes and deposits additional silicon onto the rods. Silicon produced from this process is called polycrystalline silicon and typically has impurity levels in the parts per billion range, or 1013 atoms/cm3.
The Siemens process is very expensive and accounts for a major component of the cost of the wafers used to form crystalline silicon solar cells. Upgraded metallurgical grade (UMG) silicon refers to a type of silicon with higher boron or phosphorus impurities but with other harmful impurities such as heavy metals in the parts per million range or better, and is less expensive to manufacture than high purity polysilicon. If UMG silicon, containing higher levels of boron or phosphorus impurities than the silicon produced by the Siemens process, could be used, then less expensive processes could be used to purify the metallurgical grade silicon, thereby lowering the cost of making solar cells.
Therefore, there is a need for a method of making crystalline silicon solar cells on less expensive, low purity UMG substrates.